1. Field of the Invention
This invention relates to injection laser diodes. Still more particularly, this invention relates to a laser diode system comprising two stripe-geometry laser diodes operated in a crossed-junction configuration.
2. Description of the Prior Art
Semiconductor lasers were first developed in the early 1960's. The basic structure of a semiconductor laser is that of a P-N junction diode. Under appropriate conditions, charge carriers from each side of the junction combine within the junction to produce the laser light emission. Although semiconductor lasers may be excited by the use of electron beams as well as by optical pumping, by far the most common method of excitation is by charge carrier injection produced by forward biasing of the semiconductor diode by an external electric potential which drives both electrons and holes into the junction. The rather complex theory of semiconductor laser operation may be briefly summed up by stating that the energy levels of a pure semiconductor crystal are grouped in bands, each band representing a vast number of closely spaced levels. There are gaps between these bands--ranges of forbidden energy. At a low temperature, only the lowest bands of the semiconductor crystal atoms are filled with electrons and filled completely. The highest filled band is called the valence band; the lowest empty band, the conduction band, Excitation of such a crystal represents the transfer of an electron from the valence band into the conduction band and the creation of a hole in the valence band. Excitation occurs, for example, when light is absorbed in the semiconductor. Conversely, when an electron in the conduction band unites with a hole in the valence band, emission of radiation takes place. The thickness of the junction region from which the light originates is only a few micrometers.
When a junction diode is prepared for laser operation, its front and back surfaces normally are cut perpendicular to the plane of the junction and parallel to each other. These end surfaces form the terminating mirrors of the laser. The side surfaces are usually offset by a small angle or are roughly finished to avoid regeneration of radiation in an undesired direction. As the charge carrier injection into the semiconductor junction is increased, the emitted light intensity varies linearly until a threshold is reached, at which point the intensity of the emitted light increases rapidly, the radiation pattern becomes highly directional, and the spectral width of the emitted radiation is narrowed. These phenomena are characteristic of the onset of stimulated emission.
The above described simple device commonly has a random mode distribution, both longitudinally and transversally. This random mode distribution renders the simple device essentially unusable for any of the multitude of laser applications which require laser output mode stability. Indeed, in the simple injection laser diode, the mode spectra commonly will not reproduce from run to run for a given device. This difficulty was overcome by the development of the stripe-geometry injection laser diodes in the late 1960's. In these devices, the active region of the junction is confined to a relatively narrow stripe underneath one of the metallic contacts driving the device. The stripe-geometry devices are much more well-behaved than the simple geometry devices and can produce repeatable high resolution mode spectra. Nevertheless, even the stripe-geometry injection laser diode is incapable of stable single mode operation.
Stable single mode operation is extremely important because the greatest problem in use of the injection laser is in coupling the laser output effectively into either lenses or optical fibers. To this end, most such power starved systems require single gaussian mode operation for smallest spot size and beam divergence. Laser diodes, since they use flat crystal facets for integral internal mirrors, operate in a naturally unstable cavity configuration. Typically, a single heterostructure diode operates with about twenty milliwatts per mode, giving literally hundreds of modes in a typical ten-watt pulse. Moreover, peak power operation is limited to an average of one watt per mil of junction because statistical fluctuations cause large, instantaneous variations in the local facet power density. Continuous wave (CW) injection laser diodes operate in a single transverse mode only if an aperture of from about two to about five micrometers is provided in the cavity. This limits the peak power to about three millowatts per micrometer, or only about five to about fifteen milliwatts in any commercial diode. Making the diodes larger or wider gives additional mode control problems. Coupling diodes together along their junction planes as amplifiers by focusing one diode into another or by close coupling has inherent feedback noise problems and an unfortunate sub micron alignment maintenance requirement. In addition, injection laser diodes have such high gain that saturation occurs in only a few tens of microns with loss of power efficiency in the usual high power lasers. Greater efficiency can only be obtained in a longer diode by providing a diverging light configuration which is not possible in any published diode configuration. The other requirement usually made for high power lasers is a symmetrically expanding output beam which is not possible in any but the lowest power CW diode lasers.